1. Field of the Invention
The present invention relates to a pressure regulating valve for an inkjet printer.
2. Discussion of the Background
Inkjet printers that are used commercially for making large posters, banners, etc., consume an enormous amount of ink per unit time. Consequently, a large ink reservoir (ink tank, etc.) is provided on a device main body side, and a resin tube, etc., is used to connect the ink reservoir and a print head arranged in a carriage for supplying the ink. To prevent leakage of the ink without inhibiting a discharge action of the ink by the print head, a supply pressure of the ink to the print head is required to be set to a marginally negative pressure of a predetermined value by which a meniscus is formed on an ink fluid surface in a nozzle. Well known systems such as a hydraulic head system and a pressure regulating valve system are available for achieving it. In the hydraulic head system, a structure is used in which a fluid level of the ink in the ink reservoir is somewhat lower than a height of the nozzle. In the pressure regulating valve system, a pressure regulating valve is arranged in an ink supply channel, connecting the ink reservoir and the print head, and a pressure on a print head side is set to a marginally negative value than that of the pressure regulating valve.
The pressure regulating valve used in the pressure regulating valve system includes, as key components, a negative pressure generating unit mounted on one side face of a casing that serves as a base and connected to the print head, and a sealing valve mounted on other side face of the casing across a partition wall and connected to the ink reservoir. The negative pressure generating unit is connected to the print head and includes a negative pressure chamber formed by a first depressed portion on one side face of the casing and a flexible member that covers the first depressed portion, a pressure receiving member that is bonded to the flexible member, and a pressure receiving and biasing unit that biases the pressure receiving member outwardly from the negative pressure chamber. The sealing valve includes a pressure chamber formed by a second depressed portion provided coaxially with the first depressed portion on the other side face of the casing and a shielding member that covers the second depressed portion and that connects with the ink reservoir, a valve element that is arranged in the pressure chamber and that engages with and disengages from the pressure receiving member via an opening that communicates with the negative pressure chamber, and opens and closes the opening, and a valve element biasing unit that biases the valve element in a valve closing direction (for example, refer to Japanese Patent Application Laid-open No. 2007-76373).
In the pressure regulating valve having such a structure, the pressure receiving member, biased by the pressure receiving and biasing unit, causes the flexible member to stretch outwardly, thus expanding a volume of the negative pressure chamber. Due to this, the ink in the negative pressure chamber is siphoned from the print head that is connected to the negative pressure chamber, producing a marginally negative pressure. When the ink is discharged from the print head, the ink in the negative pressure chamber is consumed, and the negative pressure increases, due to a pressure difference with the atmospheric pressure, the flexible member buckles inward into the negative pressure chamber, and is opposed by an urging force. Due to this, the pressure receiving member is caused to move, and push against the valve element, causing the sealing valve to close. Due to this, the ink in the pressure chamber is supplied to the negative pressure chamber through the opening, and due to these actions, an amount of ink that corresponds to an amount of ink consumed is supplied to the negative pressure chamber, and in addition, an exit pressure of the pressure regulating valve, that is, a pressure in the ink supply channel on the print head side is maintained at a marginally negative pressure of a predetermined value.
In such a pressure regulating valve for an inkjet printer, to realize a function of preventing leakage of the ink from the nozzle without inhibiting the discharge action of the ink by the print head, a pressure of the ink that is delivered from the ink reservoir to the print head should be adjusted to a predetermined negative pressure (hereinafter, “head supply pressure”) of a narrow range of negative pressure of −0.5 kilopascal (kPa) to −4 kPa. The main factors that regulate a relationship between the head supply pressure and an ink supply are a pressure receiving surface area of the flexible member that is subjected to the pressure difference with the atmospheric pressure, the urging force of the pressure receiving and biasing unit, a pressure receiving surface area of the valve element that is subjected to a fluid pressure of the ink stored in the ink reservoir, and the urging force of the valve element biasing unit.
The urging forces of the pressure receiving and biasing unit and the valve element biasing unit, which are typically springs, vary according to individual differences of the biasing units and the way they are assembled. It is extremely difficult to stably construct (manufacture) a biasing unit with a reduced urging force. In such a case, when the head supply pressure drops below the range (for example, to −5 kPa), the meniscus in the nozzle is lost and ink discharge stops. Furthermore, a minute amount of air in the ink is formed into bubbles in the negative pressure chamber, and the head supply pressure varies, leading to unstable ink discharge. As a measure to prevent such a problem, if tolerance levels (margins) are increased for the variations in the urging forces produced by the pressure receiving and biasing unit and the valve element biasing unit or for the variations in the fluid pressure of the ink stored in the ink reservoir, a surface area of the flexible member must be increased, resulting in a loss of compactness of the pressure regulating valve.